1. Field of the Invention
This invention relates to a method of fracturing subterranean formations penetrated by a well bore utilizing carbon dioxide based fluids. More particularly, this invention relates to a method of fracturing a subterranean formation with a substantially anhydrous two-phase fluid.
2. Description of the Prior Art
The treatment of subterranean formations penetrated by a well bore to stimulate the production of hydrocarbons therefrom or the ability of the formation to accept injected fluids has long been known in the art. One of the most common methods of increasing productivity of a hydrocarbon-bearing formation is to subject the formation to a fracturing treatment. This treatment is effected by injecting a liquid, gas or two-phase fluid which generally is referred to as a fracturing fluid down the well bore at sufficient pressure and flow rate to fracture the subterranean formation. A proppant material such as sand, fine gravel, sintered bauxite, glass beads or the like can be introduced into the fractures to keep them open. The propped fracture provides larger flow channels through which an increased quantity of a hydrocarbon can flow, thereby increasing the productive capability of a well.
A traditional fracturing technique utilizes a water or oil-based fluid to fracture a hydrocarbon-bearing formation.
Another successful fracturing technique has been that known as "foam fracturing". This process is described in, for example, U.S. Pat. No. 3,980,136. Briefly, that process involves generation of a foam of a desired "Mitchell quality" which then is introduced through a well bore into a formation which is to be fractured. The composition of the foam is such that the Mitchell foam quality at the bottom of the well is in the range of from about 0.53 to 0.99. Various gases and liquids can be used to create the foam, but foams generally used in the art are made from nitrogen and water, in the presence of a suitable surfactant. The pressure at which the foam is pumped into the well is such that it will cause a fracture of the hydrocarbon-bearing formation. Additionally, the foam comes out of the well easily when the pressure is released from the well head, because the foam expands when the pressure is reduced.
Yet another fracturing technique has been that which utilizes a liquefied, normally gaseous fluid. U.S. Pat. No. 3,195,634, for example, discloses a method for treating a subterranean formation penetrated by a well bore with a composition comprising a liquid-liquid mixture of carbon dioxide and water. The carbon dioxide is present in an amount equivalent to from about 300 to about 1500 SCF at 80.degree. F. and 14.7 psia per 42 gallons of water. The composition is injected into the formation under sufficient pressure to fracture the formation. The composition can include gelling agents and proppant materials. Upon pressure release at the well head, the liquid carbon dioxide vaporizes and flows from the formation.
U.S. Pat. No. 3,310,112 discloses a method of fracturing a subterranean formation penetrated by a well bore comprising introduction of a mixture of liquid carbon dioxide and a propping agent slurried in a suitable vehicle into the well bore at a pressure sufficient to fracture the formation. The liquid carbon dioxide is present in an amount sufficient to provide at least five volumes of carbon dioxide per volume of slurried propping agent. After injection of the liquid carbon dioxide containing the propping agent, the pressure on the well bore is released. The liquid carbon dioxide normally is heated sufficiently by the formation that upon pressure release, the liquid carbon dioxide returns to a gas phase. A substantial portion of the carbon dioxide then leaves the well and forces or carries out with it an appreciable amount of the oil or aqueous vehicle utilized to transport the proppant.
U.S. Pat. No. 3,368,627 discloses a method of treating a formation penetrated by a well bore which consists essentially of injecting down the well bore a fluid azeotropic mixture which has a critical temperature sufficiently high or a critical pressure sufficiently low to remain a liquid at the temperature and pressure existing during injection and treatment of the formation. The fluid mixture has critical properties such that a substantial portion of the injected fluid is converted to a gas upon a release of the pressure applied to the liquid during injection into the formation. The fluid mixture consists essentially of carbon dioxide and at least one C.sub.2 to C.sub.6 hydrocarbon.
U.S. Pat. No. 3,664,422 discloses a method of treating a subsurface earth formation penetrated by a well bore comprising injection of a liquefied gas together with a gelled alcohol into the formation at a pressure sufficient to fracture the formation. The liquefied gas is returned from the formation by vaporization following pressure reduction on the well bore. The gelled alcohol is removed by vaporization during subsequent production from the well leaving only the broken gelling agent in the formation.
It would be desirable to provide a method by which a viscous fluid can be created from carbon dioxide and a substantially anhydrous fluid which is stable over a broad temperature range and is capable of carrying high concentrations of proppant into a subterranean formation.